Many shapes and objects are formed from sheet or plate stock with a variety of different types of machine tools. For example, punch presses can be used for repeatedly forming identical shapes and objects in sheet metal and relatively thin plate. With a punch press, dies corresponding to the shape of the pattern to be formed are directed against the sheet stock such that the desired object or pattern is formed in the material and removed therefrom when the dies retract. Such punch presses are shown, for example, in U.S. Pat. Nos. 3,800,643 and 4,030,391, both assigned to the present assignee.
A high temperature torch can be used to melt the material and cut a shape in or a part from the sheet or plate. One such torch operates by ionizing a column of gas with an electric arc to produce temperatures up to 30,000.degree. F. A high velocity, high temperature plasma stream and electric arc strike the workpiece with the heat serving to instantly melt the material, and the high velocity stream carrying the removed material, or slag, away. Such plasma torch machine tools are disclosed in U.S. Pat. Nos. 4,063,059 and 5,763,852, both assigned to the present assignee.
In still further types of machine tools, a laser beam is employed for melting the material and forming the cut. With a typical laser-equipped machine tool, a laser beam is directed through a focusing lens or optic to direct the focal point of the laser beam above, on or below the surface of the material to be cut. The laser beam is directed from the focusing optic through a nozzle positioned immediately above the workpiece, and a pressurized gas is directed through the nozzle, typically coaxially with the laser beam, to assist making the cut. The pressurized gas interacts with the laser beam and material, to facilitate the cutting process, and creates a gas stream which carries the removed material away from the cut. It is advantageous to use gas pressures of several hundred pounds per square inch to cut some types of material such as stainless steel.
When using a laser-equipped cutting machine tool it is advantageous that means be provided to control the position of the nozzle at a variable preset distance above the surface of the material to be cut. A means commonly used is a system wherein a cutting head includes a manually adjustable ring surrounding the cutting nozzle. The ring includes a number of downward pointing ball bearings projecting a small distance from the ring. To cut, the cutting head is lowered until the ball bearings contact the material. The ball bearings then ride on the surface of the material supporting the cutting head and maintaining the position of the nozzle at the preset distance above the material. An advantage of such a system is that it will work for cutting nonmetallic materials. A disadvantage is the larger outside diameter of the cutting head increases the chance of interfering with and colliding with previously cut parts which may tip on the supporting surface extending an edge above the surface of the sheet. Such a system is also prone to causing scratches in the surface of the material which may cause problems when the final finish is applied to the part.
Another known means is to provide a servo axis to move and position the nozzle above the material to be cut, and to equip the cutting head such that the nozzle is used as an element of a capacitivie sensor system. Such a system is non-contact and thus will not scratch the sheet. Such a system can be made with a small outside diameter minimizing the possibility of interfering with previously cut, tipped up parts. Such a system has a disadvantage in that it will not work for cutting nonmetallic materials.
Another commonly used means is to provide a servo axis to move and position the nozzle above the material to be cut, and to equip the cutting head with a foot which interacts with a feedback means to the servo. Such a system has the advantage that it will work with metallic and nonmetallic materials. Such a system has the disadvantages that it may scratch the sheet. It is also larger than a capacitive sensor system and more likely to collide with previously cut, tipped up parts and be damaged thereby.
When using laser-equipped cutting machine tools it is advantageous to utilize optics with different focal lengths to cut various thicknesses of material. The focal length of the optic contributes to the diameter of the focal spot and thus the energy density, watts per square unit of area, at the focal spot. Shorter focal length optics create smaller focal spots having higher energy densities. The focal length of the optic also contributes to depth of focus of the focal spot, with longer focal lengths having greater depth of focus. Shorter focal length optics are advantageous for cutting thinner materials, while longer focal length optics are advantageous for cutting thicker material. Providing means to use optics with different focal lengths extends the range of material thicknesses which can be cut under optimum conditions.
A known apparatus for accomplishing such objectives is to provide a cutting head with multiple positions or slots in which focusing optics can be placed. Focal optics are commonly available in more or less standard focal lengths 2.5", 3,75", 5", 7.5", 10", 12.5", etc. For example, if it has been determined that a laser-equipped cutting machine is to cut a range of material thicknesses which can best be cut with focal lengths between 2.5" and 5", a cutting head might be supplied which has three slots; one for a 2.5" focal length lens, one for a 3.75" focal length lens, and one for a 5" focal length lens. The lenses would be mounted in a holder which is in turn adapted to fit in a slot in the cutting head. Multiple lens holders can be supplied providing the ability to quickly change form one focal length lens to another. Only one focal length lens is used in the cutting head at any one time. The other unused lens slots are filled with blank holders which serve only to seal the laser beam path in the cutting head.
When using a laser-equipped cutting machine tool it is also advantageous that the focusing optic be independently adjustable in position relative to the material being cut while the position of the cutting nozzle is maintained at a fixed distance above the surface of the material. The optimum position of the focal spot above the surface, on the surface, or below the surface of the material, changes with the type and thickness of material cut, and during piercing cycles.
It is also advantages during piercing of thick materials to change the position of the focusing optic relative to the surface of the material. Lowering the focusing optic while piecing drives the laser beam focal spot into the material hastening the piercing process.
One approach for changing the position of the focusing optic is to adapt the cutting head for vertical movement of the focusing lens holder and lens carrier. The lens carrier is adapted to move vertically and is guided by ways or bearings on round rods. An air cylinder or a translation screw, driven by a servo or stepper motor, is adapted to position the lens carrier upon command from the machine control.
Problems are encountered when adapting a cutting head having a vertical moving lens and/or lens carrier for use with high pressure cutting assist gas. The high-pressure gas is usually introduced into a chamber which is closed at the upper end by the focusing lens and at the lower end by the cutting nozzle. The gas pushes up against the lens and lens holder with a force "F" equal to the gas pressure in units of force per square unit of area times total square unit area of the lens and lens holder. This force can be substantial. The lens driving mechanism must be sized to resist this force. This leads to larger sized translation screws, motors and cylinders which are detrimental to designing a lightweight system which is advantageous in fast moving, high performance machines. Further, if a single translation screw or cylinder is used to drive the lens, the resultant force is an off center load between the lens and the screw, which complicates providing adequate guidance to the lens to prevent it from deflecting or canting out of square relative to the laser beam.
On laser-equipped machine tools it is also advantageous to use different types of cutting assist gasses. For example nitrogen may be most advantageous for use in cutting one type of material, with oxygen being the best alternative for another. All known lasers have cutting heads including a nozzle through which the laser beam and pressurized gas pass, and the orifice of the nozzle is relatively small. When the type of pressurized gas is to be changed, the previous gas must first be purged from the cutting head gas chamber through the small diameter nozzle orifice. The small diameter of the orifice can make the purging process quite slow. If the gas is not adequately purged, the quality of the cut may suffer for a time until the old assist gas is adequately purged during cutting.